1. Field of the Invention
This invention relates generally to chemical mechanical planarization apparatuses, and more particularly to methods and apparatuses for improved edge performance in chemical mechanical polishing applications by controlling airflow beneath a substrate.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform Chemical Mechanical Planarization (CMP) operations, including polishing, buffing and substrate cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. Patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization. Further applications include planarization of dielectric films deposited prior to the metallization process, such as dielectrics used for shallow trench isolation of poly-metal features.
Typically CMP systems implement a belt, orbital or brush operation in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of substrate. The pad itself is typically made of polyurethane material or other suitable material and may be backed by a supporting belt, for example a stainless steel belt. In operation a slurry material is applied to and spread across the surface of the polishing pad or belt. As the belt or pad covered in slurry rotates, a substrate is lowered to the surface of the pad and is polished.
FIG. 1 illustrates an exemplary prior art CMP system 10. The CMP system 10 in FIG. 1 is a belt-type system, utilizing a polishing pad 18 mounted on two drums 24 which drive the polishing pad 18 in a rotational motion as indicated by rotation directional arrows 26. A substrate 12 is mounted on a carrier head 14, which is rotated in direction 16. The rotating substrate 12 is then applied against the polishing pad 18 with a force to accomplish a CMP process. Some CMP processes require significant force F to be applied. A platen 50 is provided to stabilize the polishing pad 18 and to provide a solid surface onto which to apply the substrate 12. Slurry 28 composing of an aqueous solution such as NH4OH or DI water containing dispersed abrasive particles is introduced upstream of the substrate 12. The process of scrubbing, buffing and polishing of the surface is achieved by using the polishing pad 18. Typically, the polishing pad 18 is composed of porous or nonporous or fibrous materials and lacks fix abrasives. The polishing pad is grooved for slurry transportation under the substrate. The polishing pad 18 contains grooves and micropores that transport slurry 28 under the substrate 12 to be polished.
FIG. 2A provides a cross sectional view of the prior art CMP system 10 discussed in FIG. 1 above. The carrier head 14 may contain a retaining ring 32 that surrounds and retains the substrate 12 during processing. Air pressure supplied through the platen 50 provides support to the back of the polishing pad 18.
FIG. 2B is a detailed view of a conventional platen configuration 80. The illustration provides a top view of the polishing pad 18 and the platen 50 positioned below the carrier head as seen in FIG. 1. Often, the platen 50 includes air holes 55 to provide upward air pressure to support the polishing pad 18 which rotates beneath the surface of the substrate being polished. In prior art platen design, air escapes 118 allow air to uncontrollably and randomly and non-uniformly leak over different surface regions defined between the area above the platen and below the polishing pad 18. In the case of 300 mm or larger wafers, non-uniformity of air pressure from uncontrollable leakage is more noticeable due to the larger platen diameter relative to the polishing pad 18 width. The air escapes 118 being of varying length shown in FIG. 2 note that air escapes 118 perpendicular to the polishing pad 18 implemented in a belt format is of shorter distance than at other angles (e.g., 45 degrees) allowing greater flow in the regions of shorter distance. Although several straight lines are illustrated to show some paths that the air can randomly escape over the surface of the platen cover 22, it should be understood that air can escape from the platen 50 and over the platen cover 22 at any location around the periphery of the platen 50.
In summary, non-uniform leakage of fluid beneath the polishing pad 18 provides an uneven polishing surface for the substrate creating an undesirable non-uniform removal. Uncontrolled leakage of air supplied to the backside of the polishing pad 18 on CMP systems creates an additional burden of greater facility requirements and higher operational cost.
There is a need therefore for a platen design that provides uniform pressure beneath the polishing surface by uniformly distributing and otherwise controlling fluid escape.